Conventionally, a supercharging system is known in which two superchargers are arranged about one engine. For example, in a parallel supercharging system, two superchargers compress intake air flowing in two intake paths. In a series supercharging system, two superchargers compress intake air flowing in one intake path. The Patent Literature 1 discloses an engine having a so-called sequential twin turbo which is a form of the series supercharging system.
A supercharger of variable capacity type is also known in which exhaust gas is introduced to a turbine constituting the supercharger while controlling flow rate suitably so as to improve supercharging efficiency. A parallel supercharging system in which two superchargers compress intake air flowing in two intake paths and at least one of the superchargers is variable capacity type is defined as a variable parallel supercharging system.
Conventionally, in control of capacity in a variable capacity type supercharger, feedback control is performed with supercharging pressure detected by a supercharging pressure sensor. However, the supercharging pressure is indirect physical quantity about the action of the supercharger and is disadvantageous because the control of capacity of the supercharger cannot be performed accurately. Especially, the variable parallel supercharging system having the variable capacity type superchargers is disadvantageous because the control of capacity cannot be performed accurately further.
Patent Literature 1: the Japanese Patent Laid Open Gazette Hei. 5-288111